Injection Molding of Plastics & TPEs: Chemical Properties & Material Testing
Introduction to Injection Molding
Injection molding is the most widely used manufacturing process for producing plastic parts at high volume. Molten polymer is injected under high pressure into a precision-machined steel or aluminum mold, where it solidifies to the mold geometry. The process can produce complex three-dimensional shapes with tight tolerances, an excellent surface finish, and minimal post-processing.
Both rigid thermoplastics (ABS, PP, PC, PA, POM) and thermoplastic elastomers (TPEs) are injection moldable, but their processing requirements and material behaviours differ significantly. Understanding these differences is essential for the automotive, consumer goods, medical device, and packaging industries.
What Are Thermoplastic Elastomers (TPEs)?
Thermoplastic elastomers are a class of copolymers or polymer blends that exhibit rubber-like elasticity at service temperatures but can be processed as thermoplastics—melted and molded repeatedly without chemical crosslinking. Major TPE families include:
- SBS/SEBS (Styrenic block copolymers): Soft-touch grips, footwear, overmolds
- TPO/TPV (Polyolefin-based): Automotive seals, bumpers, weather strips
- TPU (Polyurethane-based): Hose, cable jacketing, medical tubing
- PEBA (Polyether block amide): Sports equipment, medical catheters
- TPC (Copolyester-based): Automotive connectors, industrial hose
Key Processing Differences: Rigid Plastics vs. TPEs
Parameter | Rigid Thermoplastics | TPEs |
Melt viscosity | Generally higher | Lower (especially TPU, SEBS) |
Mold temperature | Moderate to high | Lower for most grades |
Shear sensitivity | Moderate | High (especially TPUs) |
Drying requirement | Many (PA, PC, PET) | Yes (TPU, PEBA critical) |
Gate design | Standard | Larger gates preferred |
Demolding | Stiff ejection | Flexible, requires care |
Overmolding with TPEs
One of the most valuable applications of TPEs in injection molding is overmolding—the process of molding a TPE layer over a rigid thermoplastic substrate to create a soft-touch surface, grip region, or seal. Successful overmolding requires chemical or mechanical adhesion between the TPE and substrate:
- Chemical adhesion: Best achieved with compatible polymer families (e.g., SEBS over PP; TPU over PC/ABS)
- Mechanical interlocking: Through holes, undercuts, or textured surfaces
Material Testing for Injection-Molded Plastics and TPEs
Quality assurance for injection-molded parts requires both incoming material testing and finished-part testing:
Raw Material Testing
- Melt flow index (ASTM D1238) — verifies processing viscosity
- Moisture content (Karl Fischer or loss-on-drying) — critical for hygroscopic materials
- Density (ASTM D792)
- Colour and appearance (spectrophotometry)
Mechanical Property Testing of Molded Parts
- Tensile strength and elongation (ASTM D638 for rigid; ASTM D412 for TPEs)
- Hardness (Shore A for TPEs; Shore D or Rockwell for rigid plastics)
- Flexural modulus (ASTM D790)
- Impact resistance (ASTM D256 Izod; ASTM D4812 unnotched Charpy)
Adhesion Testing for Overmolded Parts
- Peel strength (ASTM D1876 or custom fixture)
- Shear adhesion
- Cross-hatch adhesion tape test (ASTM D3359)
Conclusion
Injection molding remains a cornerstone of modern manufacturing, enabling the efficient production of complex plastic components with high precision and consistency. The ability to process both rigid thermoplastics and thermoplastic elastomers (TPEs) expands design flexibility, allowing engineers to combine structural strength with elasticity, comfort, and sealing functionality.
Understanding the differences in processing behaviour, material properties, and adhesion requirements—especially in applications like overmolding—is essential for achieving optimal performance. Through proper material selection, process control, and comprehensive testing, manufacturers can ensure high-quality, reliable parts that meet demanding application requirements across industries.
Why Choose Infinita Lab for Injection Molding Material Testing?
Infinita Lab provides comprehensive testing services for injection-molded plastics and TPEs, from raw material characterisation and melt rheology to finished-part mechanical testing and overmold adhesion evaluation. Our accredited network delivers fast turnaround and expert technical support.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote
Frequently Asked Questions (FAQs)
What is the most important processing parameter to control for TPU injection molding? Moisture content is critical for TPU. Inadequately dried TPU hydrolyzes at melt temperature, causing molecular weight degradation, surface defects (splay, bubbles), and dramatically reduced physical properties. TPU must be dried to below 0.02% moisture before molding.
How is Shore hardness selected for TPE overmold applications? Shore A hardness governs the soft-touch feel. Most grip and ergonomic overmolds use 40–60 Shore A. Sealing applications may use 20–40 Shore A for compliance. Structural TPE applications may use 60–80 Shore A or harder.
Can all rigid thermoplastics be overmolded with TPEs? No. Adhesion depends on chemical compatibility between the TPE and substrate. PP substrates bond well with TPO and SEBS. ABS and PC bond well with TPU. Polyamide bonds well with PEBA. Incompatible combinations require mechanical interlocking or adhesion promoters.
What causes flash defects in TPE injection molding? Flash (thin polymer film at parting lines) in TPEs is caused by the low melt viscosity of TPEs flowing into parting line gaps. It is addressed by optimizing injection pressure, improving parting line fit, and selecting higher-viscosity TPE grades where possible.
What ASTM standards apply to testing injection-molded TPE parts? ASTM D412 covers tensile testing of rubber and TPEs. ASTM D2240 covers Shore A and D hardness. ASTM D395 covers compression set. ASTM D1938 covers tear resistance. ASTM D573 covers heat aging of rubber and TPEs.
